New recent multimedia applications, such as high-definition audio/video streaming, require wireless transmission of uncompressed video at high data rate of about several Gbps (Gigabits per second), with low latency.
High definition (HD) video generally refers to a sequence of 60 Hz video frames with 1920 vertical lines and 1080 horizontal lines corresponding to 1920×1080 pixels of 24 bits. A video wireless communication system that supports such HD video requires a channel or communication link bandwidth of about 3 Gbps to support video data only.
Such rate is not achievable in current 802.11 wireless communication systems using the 2.4 GHz and 5 GHz radio bands. To overcome the limits of these radio bands, higher frequencies, for instance the 57-66 GHz millimeter-wave unlicensed spectrum, referred to as 60 GHz millimeter wave technology, are used.
60 GHz-based communication systems are widely studied (e.g. IEEE 802.11 Task Group; IEEE 802.15.3c standard; Wireless HD; WiGiG; etc.) and the research community proposes several solutions and methods to transport the audio and video applications with a desired quality of service (QoS).
In the wireless communication systems, before a video stream is transmitted, connection setup and communication link bandwidth reservation are conducted. Ideally, sufficient link bandwidth can be allocated and the video stream can be transmitted smoothly after stream set-up control.
However, the 60 GHz millimeter wave technology is highly sensitive to perturbations (shadowing or interference) or fading phenomena making the quality of the wireless link dynamically change over time. For example, for 60 GHz wireless channels with beam-formed transmissions, the communication link can be affected even by a person moving which appears as an unexpected obstacle on a transmission path. In these cases, the degradation of the communication link requires for example changing the channel coding (for instance by increasing the redundancy data) or/and the type of modulation of video data. This results in the initial allocated link bandwidth being no longer sufficient for video transmission.
Spatial diversity is a well-known approach that improves the quality and reliability of a wireless link, and thus brings robustness when communicating information on a wireless communication link. Spatial diversity is achieved by utilizing a plurality of transmission paths for transmitting the same information in a wireless communication system between two communicating stations. Alternative transmission paths may result for example from the use of several antennas or from the reflection of the data signal on various objects.
In practice, if a transmission of information fails through one transmission path due to the presence of an unexpected obstacle for example, the same information is still likely to reach the intended wireless receiving station through an alternative transmission path.
However, the spatial diversity requires duplicating the same information on a plurality of transmission paths, thus consuming communication network bandwidth and limiting the useful bandwidth allocated to the source data.
Therefore, using spatial diversity for conveying video stream between a wireless transmitting station (source device) and a wireless receiving station (destination device) would result in decreasing the video stream resolution so that it fits with the available link bandwidth, thereby reducing the quality of video rendering.
One well-known approach to decrease the video stream resolution is based on performing a video component removing or dropping operation, referred herein to as “sub-sampling”.
Sub-sampling generally consists in dropping some colour information or components, such as chroma components, from the original video pixels, so as to decrease the amount of pixel information without significant visual or rendering degradation. This may be acceptable because the human vision system is less sensitive to colour (chroma components) than brightness (luma component).
In other words, the colour information of the video frames is sampled at a lower resolution.
Three main pixel information dropping profiles for sub-sampling are commonly used in video compression standards: 4:4:4, 4:2:2 and 4:2:0 corresponding respectively to a reduction of the video data of 0%, 33% and 50%. The selected profile is usually applied to the whole video frame.
However, the number of sub-sampling profiles is very small and the reduction between two levels is very high. This is not suitable to optimize the use of the available link bandwidth over the multiple transmission paths, while keeping the best high quality video rendering.
An example is given to illustrate this assertion: if a node of the wireless communication network needs to reduce the video rate by about 10% (i.e. of the useful video data), the second profile or level (i.e. 4:2:2) has to be used. This use results in a video rate reduction of 33% whereas only a reduction of 10% would be necessary. Since more video components than necessary are then removed or dropped from the original video stream, the rendering of the video is severely decreased in quality.
Another approach to achieve robust communication over unreliable communication channels such as in a lossy packet network is disclosed through the publication “Multiple Description Coding via Polyphase Transform and Selective Quantization” by W. Jiang and A. Ortega.
Data from a video source is first decomposed into two sub-sources via a polyphase transform. Each of these two sub-sources is quantized independently according to a first quantization scheme so as to provide the primary part of information of each of two communication channels. For reconstruction of the other channel in case of loss of one channel, each channel also carries information about the other channel: a coarsely quantized version of the other sub-source that is generated based on a second quantization scheme.
The quantized output from the first and second quantization schemes are multiplexed together for transmission over each communication channel. At the receiving station, if data from both channels arrives, fine quantized data of both polyphase components is then used for reconstruction of the original video source data.
Otherwise, if data from one channel is lost, one fine quantized polyphase and one coarsely quantized polyphase component are used for reconstruction of the best possible version of the original video source data.
Such a polyphase and quantization-based approach appears to be resource-demanding for the transmitting and receiving stations.